Threshold Enhancements Research Articles

Toy model to understand the oscillatory behavior in timelike nucleon form factors

To understand the oscillatory behavior exhibited in the timelike electromagnetic form factors of nucleons, we propose a toy model based on the Jost function of the $N\bar N$ pair into the timelike form factors with the help of the distorted-wave Born approximation. By constructing a simple square-well potential reflecting the final-state interaction of $N\bar N$, we naturally represent the damped oscillatory phenomenon in the timelike electromagnetic form factors of nucleons. Especially, our study reveals that the ``period" of the oscillation is approximately determined by the Yukawa interaction range $1/m_\pi$. Other possible potentials are also discussed. The threshold enhancements of the cross sections for $e^+e^-\to n\bar n$, $\Lambda\bar\Lambda$, and $\Lambda_c\bar\Lambda_c$ can also be understand within this scenario.

The Overshadowed χc1(2p) Charmoniumlike Resonance

A systematic analysis of JPC = 1++ charmoniumlike mesons with energies up to 4.0 GeV is carried out within the diabatic approach to QCD. The resulting spectrum contains only two bound states: the conventional 1p charmonium state and a D0 D̄∗0 loosely bound state which may be naturally identified with X(3872). The rest of the spectrum consists in a continuum of D0 D̄∗0 and D+D∗− scattering states, whose analysis indicates the presence of a resonance near 3.96 GeV that may be assigned to a mostly conventional 2p charmonium state. It is shown that this χc1(2p) resonance is likely overshadowed in its expected main decay channels, D0 D̄∗0 and D+D∗−, by the threshold enhancements due to the loosely bound X(3872), which may justify its lack of detection to date. Alternative discovery channels are discussed.

Why static bound-state calculations of tetraquarks should be met with scepticism

Recent experimental signals have led to a revival of tetraquarks, the hypothetical q2q̄2 hadronic states proposed by Jaffe in 1976 to explain the light scalar mesons. Mesonic structures with exotic quantum numbers have indeed been observed recently, though a controversy persists as to whether these are true resonances and not merely kinematical threshold enhancements, or otherwise states not of a true q2q̄2 nature. Moreover, puzzling non-exotic mesons are also often claimed to have a tetraquark configuration. However, the corresponding model calculations are practically always carried out in pure and static bound-state approaches, ignoring completely the coupling to asymptotic two-meson states and unitarity, especially the dynamical effects thereof. In this short paper we argue that these static predictions of real tetraquark masses are highly unreliable and provide little evidence of the very existence of such states.

Large loop-coupling enhancement of a heavy pseudoscalar from a light dark sector

The small background and the sensitivity to charged particles via a leading order loop coupling make the diphoton channel a privileged experimental test for new physics models. We propose a simple archetypal scenario to generate a sharp di-photon resonance as a result of threshold enhancements in the effective coupling between a heavy pseudoscalar particle and new vector-like leptons. We therefore study three different scenarios consistent with the current experimental limits and deviating from the Standard Model at the 2 σ level. The model also introduces a natural dark matter candidate able to match the observed dark matter abundance and comfortably respect the current direct detection constraints.

Higgs-stoponium mixing near the stop-antistop threshold

Supersymmetric extensions of the standard model contain additional heavy neutral Higgs bosons that are coupled to heavy scalar top quarks (stops). This system exhibits interesting field theoretic phenomena when the Higgs mass is close to the stop-antistop production threshold. Existing work in the literature has examined the digluon-to-diphoton cross section near threshold and has focused on enhancements in the cross section that might arise either from the perturbative contributions to the Higgs-to-digluon and Higgs-to-diphoton form factors or from mixing of the Higgs boson with stoponium states. Near threshold, enhancements in the relevant amplitudes that go as inverse powers of the stop-antistop relative velocity require resummations of perturbation theory and/or nonperturbative treatments. We present a complete formulation of threshold effects at leading order in the stop-antistop relative velocity in terms of nonrelativistic effective field theory. We give detailed numerical calculations for the case in which the stop-antistop Green's function is modeled with a Coulomb-Schr\"odinger Green's function. We find several general effects that do not appear in a purely perturbative treatment. Higgs-stop-antistop mixing effects displace physical masses from the threshold region, thereby rendering the perturbative threshold enhancements inoperative. In the case of large Higgs-stop-antistop couplings, the displacement of a physical state above threshold substantially increases its width, owing to its decay width to a stop-antistop pair, and greatly reduces its contribution to the cross section.

Control of threshold enhancements in harmonic generation by atoms in a two-color laser field with orthogonal polarizations

Threshold phenomena (or channel-closing effects) are analyzed in high-order harmonic generation (HHG) by atoms in a two-color laser field with orthogonal linearly polarized components of a fundamental field and its second harmonic. We show that the threshold behavior of HHG rates for the case of a weak second harmonic component is sensitive to the parity of a closing multiphoton ionization channel and the spatial symmetry of the initial bound state of the target atom, while for the case of comparable intensities of both components, suppression of threshold phenomena is observed as the relative phase between the components of a two-color field varies. A quantum orbit analysis as well as phenomenological considerations in terms of Baz' theory of threshold phenomena [Zh. Eksp. Teor. Fiz. 33, 923 (1957)] are presented in order to describe and explain the major features of threshold phenomena in HHG by a two-color field.

Triangle singularities and XYZ quarkonium peaks

We discuss analytical properties of partial waves derived from projection of a 4-legged amplitude with crossed-channel exchanges in the kinematic region of the direct channel that corresponds to the XYZ peaks in charmonium and bottomonium. We show that in general partial waves can develop anomalous branch points in the vicinity of the direct channel physical region. We show that this effect only occurs if masses of resonances in the crossed channel are in a specific, narrow range. We estimate the size of threshold enhancements originating from these anomalous singularities in reactions where the Zc(3900) and the Zb(10610) peaks have been observed.

Threshold enhancement phenomena in Y(4260) decays

We investigate several strong and radiative decay modes of Y(4260), by assuming that Y(4260) either is a [Formula: see text] molecular state, or has sizeable couplings with [Formula: see text] and [Formula: see text]. In such ansatzes, obvious threshold enhancements or narrow cusp structures appear quite naturally without introducing a genuine resonance. And we emphasize that the radiative decay modes may be useful for studying [Formula: see text]S-wave scattering.

Exploring the threshold behavior and implications on the nature ofY(4260)andZc(3900)

By assuming that $Y(4260)$ either is a ${D}_{1}\overline{D}$ molecular state or has sizeable couplings with ${D}_{0}{\overline{D}}^{*}$ and ${D}_{1}^{\ensuremath{'}}\overline{D}$, we investigate several decay modes of $Y(4260)$. Under the special kinematic configurations, triangle singularity may occur in the rescattering amplitude, which will significantly change the threshold behavior. Obvious threshold enhancements or narrow cusp structures appear quite naturally without introducing a genuine resonance. We emphasize that the radiative decay modes may be useful for studying ${D}^{(*)}\overline{D}$ $S$-wave scattering.

Observation and study of the baryonicB-meson decaysB→D(*)pp¯(π)(π)

We present a study of ten B-meson decays to a D(*), a proton-antiproton pair, and a system of up to two pions using BaBar's data set of 455x10^6 BBbar pairs. Four of the modes (B0bar -> D0 p anti-p, B0bar -> D*0 p anti-p, B0bar -> D+ p anti-p pi-, B0bar -> D*+ p anti-p pi-) are studied with improved statistics compared to previous measurements; six of the modes (B- -> D0 p anti-p pi-, B- -> D*0 p anti-p pi-, B0bar -> D0 p anti-p pi- pi+, B0bar -> D*0 p anti-p pi- pi+, B- -> D+ p anti-p pi- pi-, B- -> D*+ p anti-p pi- pi-) are first observations. The branching fractions for 3- and 5-body decays are suppressed compared to 4-body decays. Kinematic distributions for 3-body decays show non-overlapping threshold enhancements in m(p anti-p) and m(D(*)0 p) in the Dalitz plots. For 4-body decays, m(p pi-) mass projections show a narrow peak with mass and full width of (1497.4 +- 3.0 +- 0.9) MeV/c2, and (47 +- 12 +- 4) MeV/c2, respectively, where the first (second) errors are statistical (systematic). For 5-body decays, mass projections are similar to phase space expectations. All results are preliminary.

Formula omitted]-Nucleus optical potential and possible [formula omitted] bound states

Starting from a recent model of the η′N interaction, we evaluate the η′-nucleus optical potential, including the contribution of lowest order in density, tρ/2mη′, together with the second-order terms accounting for η′ absorption by two nucleons. We also calculate the formation cross section of the η′ bound states from (π+,p) reactions on nuclei. The η′-nucleus potential suffers from uncertainties tied to the poorly known η′N interaction, which can be partially constrained by the experimental modulus of the η′N scattering length and/or the recently measured transparency ratios in η′ nuclear photoproduction. Assuming an attractive interaction and taking the claimed experimental value |aη′N|=0.1 fm, we obtain an η′ optical potential in nuclear matter at saturation density of Vη′=−(8.7+1.8i) MeV, not attractive enough to produce η′ bound states in light nuclei. Larger values of the scattering length give rise to deeper optical potentials, with moderate enough imaginary parts. For a value |aη′N|=0.3 fm, which can still be considered to lie within the uncertainties of the experimental constraints, the spectra of light and medium nuclei show clear structures associated to η′-nuclear bound states and to threshold enhancements in the unbound region.

Study ofψ(2S)decays toγpp¯,π0pp¯, andηpp¯, and search forpp¯threshold enhancements

The decays of psi(2S) into gamma p bar{p}, pi^0 p bar{p} and eta p bar{p} have been studied with the CLEO-c detector using a sample of 24.5 million psi(2S) events obtained from e^+e^- annihilations at sqrt{s} = 3686 MeV. The data show evidence for the excitation of several N^* resonances in p pi^0 and p eta channels in pi^0 p bar{p} and eta p bar{p} decays, and f_2 states in gamma p bar{p} decay. Branching fractions for decays of psi(2S) to gamma p bar{p}, pi^0 p bar{p} and eta p bar{p} have been determined. No evidence for p bar{p} threshold enhancements was found in the reactions psi(2S)-> X p bar{p}, where X = gamma, pi^0, eta. We do, however, find confirming evidence for a p bar{p} threshold enhancement in J/psi-> gamma p bar{p} as previously reported by BES.

Very BroadX(4260)and the Resonance Parameters of theψ(3D)Vector Charmonium State

We argue that the X(4260) enhancement contains a wealth of information on 1-- cc spectroscopy. We discuss the shape of the X(4260) observed in the Okubo-Zweig-Iizuka-forbidden process e+e-→π+π-J/ψ, in particular, at and near vector charmonium resonances as well as open-charm threshold enhancements. The resulting very broad X(4260) structure does not seem to classify itself as a 1-- cc resonance, but its detailed shape allows us to identify new vector charmonium states with higher statistics than in open-charm decay. Here, we estimate the resonance parameters of the ψ(3D). Our approach also provides an explanation for the odd dip in the π+π-J/ψ data precisely at the ψ(4415) resonance.

Limits on threshold and “Sommerfeld” enhancements in dark matter annihilation

We find model-independent upper limits on rates of dark matter annihilation in galactic halos. The Born approximation generally fails, while exotic threshold enhancements akin to "Sommerfeld factors" also turn out to be baseless The most efficient annihilation mechanism involves perturbatively small decay widths that have largely been ignored. Widths that are very small compared to TeV mass scales suffice to effectively saturate unitarity bounds on annihilation rates. Bound state formation in weakly coupled theories produces small effects due to wave function normalizations. Unitarity shows the Sommerfeld factor cannot produce large changes in cross sections, and serves to identify where those approximations break down.

Formula omitted] quasibound states: [formula omitted] decay and [formula omitted]-atomic evidences

We use an N N ¯ interaction based in a constituent quark model to study possible N N ¯ quasibound states. Within this model the antiprotonium energy shifts are studied as a signal of such states. The threshold N N ¯ enhancement of J / Ψ decay is also studied, including FSI effects. They can reproduce this enhancement an other N N ¯ threshold enhancements, ruling out their interpretation as signals of bound states.

BESII Results on Hadron Spectroscopy

In this talk, I would report the new observation of X(1835), which has properties inconsistent with any know mesons, but consistent with expectations for the state that produces the strong [Formula: see text] mass threshold enhancement observed in the [Formula: see text] process at BESII. I will also review some other multiquark candidates which are indicated by [Formula: see text] and [Formula: see text] mass threshold enhancements.

Parity, charge conjugation, and SU(3) constraints on threshold enhancement inJ/ψdecays intoγpp¯andKpΛ¯

We study the threshold enhancement effects of baryon-anti-baryon systems in $J/\psi$ decays at BES using parity $P$, charge conjugation $C$, and flavor SU(3) symmetries. The $P$ and $C$ symmetries restrict the $p\bar p$ in $J/\psi \to \gamma p\bar p$ to be in a state with $C=+1$, while the $p\bar p$ in $J/\psi \to \pi^0 p\bar p$ to be with $C=-1$. Combining the $C$ and $P$ symmetries with flavor SU(3) symmetry, i.e. the CPS symmetry, we find that the $\Lambda \bar p$ system cannot be in $0^+$ and $0^-$ states in $J/\psi \to K^+ \Lambda \bar p$. We provide a consistent explanation of observation and non-observation of near threshold enhancements in $J/\psi \to K^+ \Lambda \bar p$ and $J/\psi \to \pi^0 p \bar p$, respectively. We also find that near baryon pair threshold enhancement can happen in several channels for $J/\psi$ decays and can be several times larger than that observed in $J/\psi \to K^+ \Lambda \bar p$ in some channels.

Complex, 3D Photonic Crystals Fabricated by Atomic Layer Deposition

ABSTRACTRecently we have demonstrated the potential of Atomic Layer Deposition (ALD) for the fabrication of advanced luminescent photonic crystal (PC) structures based on the inverse opal architecture.[1–3] PC's offer efficiency enhancement, decreased threshold, and other enhancements that improve phosphor performance. 3D PC structures are being extensively modeled, revealing that changes in the structures such as shifting the distribution of dielectric material can significantly improve photonic band gap (PBG) properties. For example, in the inverted “shell” structure, the width of the PBG can be increased from 4.25% to 8.6%.[4] Similarly, the PBG width can also be increased to 9.6% by formation of a non-close-packed structure.[5] Using the FDTD method, we have found that the PBG in a TiO2 non-closed packed structure can be as high as 5%. The performance of these structures depends critically on precisely and accurately placed high dielectric material. Using ALD, we have demonstrated infiltration of TiO2 films with extremely smooth surfaces (0.2–0.4 nm RMS roughness) while maintaining a high level of control over the infiltration coating thickness, enabling formation of composite infiltrated and inverse opals with nano-scale precision.Here we report progress in fabrication of multi-layered and non-close packed PCs using ALD. Two and three-layer inverse opals were formed by the deposition of thin layers of ZnS:Mn and TiO2 in stacked configuration, each exhibiting luminescence when excited by UV light. Evidence for modification of the emission characteristics by high order PBGs (gaps other than between the 2nd and 3rd bands) has been observed. In addition, non-close packed inverse opals have been formed by infiltrating heavily sintered silica opals with TiO2, etching the spheres with hydrofluoric acid, and backfilling the resulting inverse opal. Resulting structures were characterized using specular reflectance and transmission, photoluminescence, and SEM. This work demonstrates the enormous potential that ALD offers for the realization of high performance photonic crystal structures.

Sudakov resummation and finite order expansions of heavy quark hadroproduction cross sections

We resum Sudakov threshold enhancements in heavy quark hadroproduction for single-heavy quark inclusive and pair-inclusive kinematics. We expand these resummed results and derive analytical finite-order cross sections through next-to-next-to-leading order. This involves the construction of next-to-leading order matching conditions in color space. For the scale dependent terms we derive exact results using renormalization group methods. We study the effects of scale variations, scheme and kinematics choice on the partonic and hadronic cross sections, and provide estimates for top and bottom quark production cross sections.

Effects of higher-order threshold corrections in high-ETjet production

Results for higher-order threshold enhancements in high-${E}_{T}$ jet production in hadron-hadron collisions are presented. Expressions are given for the next-to-next-to-leading order (NNLO) threshold corrections to the single-jet inclusive cross section at next-to-leading logarithmic (NLL) accuracy. The corrections are found to be small for the specific choice of ${E}_{T}/2$ for the factorization and renormalization scales, and the corrected cross section shows a substantial reduction of the scale dependence. A comparison to experimental results from the Fermilab Tevatron is presented.